The Preparation and Characterization of Y- TZP/20 wt% Alumina

Journal o/'the European Ceramic Society 11 (1993) 497-507

The Preparation and Characterization of Y- TZP/20 wt% Alumina P. den Exter, A. J. A. Winnubst* & A. J. Burggraaf University of Twente, Faculty of Chemical Technology, Laboratory of Inorganic Chemistry, Materials Science and Catalysis, PO Box 217, 7500 AE Enschede, The Netherlands (Received 3 June 1992; accepted l0 July 1992)

Abstract Zirconia ( + 2-2"5 mol% yttria)/12-20 wt% alumina composite powders have been prepared by several techniques. The preparation methods were discussed in terms of powder characteristics, densification behaviour and microstructure. The densification behaviour q[ the composites depended on the crystal structure o[ the phases and on the dispersion o/ the alumina in the Y-TZP matrix. Both the spraying and coprecipitation methods resulted in an inhomogeneous dispersion and a relatively low density (95%), caused by either d(fferential sintering of alumina aggregates (coprecipitation method) or by an inhomogeneous dispersion of the alumina (spraying method). Both the acetvl acetonate and the ~-alumina methods resulted in hi,~h density (98%) ceramics in which the alumina was homogeneously dispersed. The ~-alumina method, however, required higher sintering temperatures (1450~:C) than the acetyl acetonate method. Both methods gave better results than those obtained with a commercially available powder. Verbunde aus Zirkoniumdioxid mit 2-2"5 Mol% Yttriumzusatz und zu 12-20 Gew.% aus Aluminiumoxid wurden mit Hilfe verschiedener Techniken hergestellt. Die Herstellungsmethoden wurden in Hinblick auf die Pulvereigenscha~en, das Verdichtungsverhalten und das Gef~ge besprochen. Das Verdichtungsverhalten der Verbunde hiingt yon der Kristallstruktur der beteiligten Phasen und yon der Verteilung des Aluminiumoxids in der Y-TZP-Matrix ab. Sowohl die Spriih- als auch die Koausscheidungsmethode ergaben eine inhomogene Verteilung und eine relativ geringe Dichte (95%). Dieses ist entweder auf d(fferentielles Sinterverhalten der AlumiumdioxidAggregate ( Koausscheidungsmethode ) oder auf eine inhomogene Verteilung der Aluminiumoxid- Teilchen * To whom correspondence should be addressed.

( Spriihmethode ) zuriickzu[iihren, lm Gegensatz hierzu ergaben die Acetyl-Acetonat- und die otAluminiumoxid-Methode Keramiken mit hoher Dichte, in denen die Aluminiumoxid-Phase homogen verteilt war. Die ~-Aluminiumoxid-Methode erfbrderte h6here Sintertemperaturen (1450°C) als die AcetylAcetonat-Methode. Beide Verfahren fiihrten zu besseren Ergebnissen als mit der Anwendung handelsiiblicher Pulver erzielt werden konnten. Des poudres composites en zircone ( + 2-2"5 mol% d'YzO3)/12-20wt% alumine ont btk prkparkes par diverses techniques. Les mkthodes de prkparation sont discutkes en termes de caractkristiques des poudres, de comportement h la dens(fication et de microstructure. La densOqcation des composites dkpend de la structure cristalline et de la dispersion de l'alumine dans la matrice en Y-TZP. Aussi bien la mOthode par pulv~;risation que celle par coprbcipitation rksultent en une dispersion hOtkrogkne et en une densification relativement faible (95%), provenant d'un frittage d(ffOrentiel des agrkgats d'alumine (mkthode par copr&'ipitation) ou d'une dispersion hktkrog~ne de I'alumine ( mOthode par pulw;risation ). Les mbthodes ac~;tyl-acOtonate et alumine-~ permettent d'atteindre des densitks blev~es ( 98% ) et d'obtenir une dispersion homogkne de l'alumine. La mOthode alumine-~t requiert cependant une tempkrature de frittage plus ~levOe (1450°C) que la mkthode acktyl-acktonate. Les deux mkthodes conduisent ~ des rksultats meilleurs que ceux obtenus h partir d'une poudre commerciale.

I Introduction

The high strength and fracture toughness of yttriastabilized tetragonal zirconia polycrystals (Y-TZP) is due to the phase transformation from tetragonal zirconia to the monoclinic structure during mechanical loading. For brittle composite materials con-

497 Journal of the European Ceramic Society 0955-2219/93/$6.00 © 1993 Elsevier Science Publishers Ltd, England. Printed in Great Britain

498

P. den Exter, A. J. A. Winnubst, A. J. Burggraaf

taining Y-TZP Lange I derived an equation for the increase in fracture toughness due to the stressinduced phase transformation:

Ko =

-+

2RViEc(IAGcl-A Use f ) (1 --

(1)

where K c is the critical stress intensity factor, Ko the critical stress intensity for the material without transformation phenomenon, IAGcl is the change in chemical free energy associated with the transformation, A U~e is the change in strain energy associated with the transformation, (1 - f ) is the loss of strain energy due to the loss of constraint imposed on the dispersed particles during crack extension, Vi the volume fraction of transformed particles, v¢ the Poisson's ratio, E~ the Young's modulus of the composite and R the width of the transformation zone associated with the crack. Addition of secondphase particles with a higher elastic modulus to TZP increases the elastic modulus of the composite E c and consequently the fracture toughness. 1 The replacement of a certain amount of zirconia by a second-phase decreases the volume fraction Vi. So the product E~V~ in eqn (1) determines whether an increase in fracture toughness can be expected. Lange I found that an increase in fracture toughness can only be expected when the Young's modulus of the second-phase particle is at least twice as large as the Young's modulus of the Y-TZP. This is just the case for a-alumina as a second-phase addition. The elastic modulus ofzirconia was reported to be in the range of 140 to 200 GPa. 2 The elastic modulus of alumina was reported to be 411 GPa. 3 The increase in elastic modulus due to the addition of alumina also influences the retention of the tetragonal structure. According to Lange 4 the increase in elastic modulus of the constraining composite matrix increases the strain energy, thus lowering the constrained transformation temperature. In other words, the addition of alumina to YTZP enlarges the critical grain size for transformation of tetragonal particles, which was experiment, ally confirmed by Hou et al. 5 Little experimental work has been done to investigate the effect of the addition of alumina on the mechanical properties of Y-TZP. It was found 6'7 that the addition of 20wt% alumina resulted in a significant increase in strength and toughness. Furthermore the addition of 20 wt% alumina to YTZP was found v to improve the resistance to spontaneous phase transformation under the influence of water elevated temperatures. The preparation of the above-mentioned composites has not been well described in the literature and is restricted to wet-milling zirconia and aalumina, s'9 or the hydrolysis of a homogeneous

solution of metal nitrates with a m m o n i a f Although it was claimed v'9 that a homogeneous distribution of alumina was obtained, this conclusion was not fully supported by micrographs. The starting point of the work pesented in this paper is to disperse 20 wt% of alumina in a zirconia powder matrix with preservation of the excellent sintering behaviour of the zirconia powder matrix as reported by Theunissen et al.l° This Y-TZP powder was obtained by the hydrolysis of a dilute solution of metal chlorides in a large excess of ammonia. 11 During non-isothermal sintering this Y-TZP powder reached 96% of the theoretical density at 1150°C with a ceramic grain size of 0-18~m. 1° This preparation method, therefore, will serve as a starting point for the preparation of zirconiaalumina composite powders. The alumina will be homogeneously dispersed as a metastable transition alumina. The presence of transition aluminas is a consequence of the use of wet-chemical preparation techniques and calcination temperatures which are too low to form a-alumina. It has been shown in the literature ~2'13 that during sintering the 0- to ~alumina phase transformation takes place at approximately 1240°C. This is considerably higher than the temperature at which the surrounding zirconia matrix is thought to reach a density of more than 95% of the theoretical density. ~° The 0- to aalumina phase transformation is accompanied by a considerable shrinkage of the isolated alumina particles. This shrinkage must take place without breaking the contact with the surrounding zirconia matrix. The question arises whether the use of transition alumina-containing composite powders can serve as a starting material for the preparation of YT Z P / 2 0 w t % a-alumina without increasing the sintering temperature necessary to obtain dense ceramics to a higher value than the temperature necessary to transform all 0-alumina particles (1240°C). Furthermore the question must be answered to what extent homogeneity is a n impor ttant factor, especially with respect to differential sintering of any possible alumina aggregates. In this paper several wet-chemical preparation techniques are presented which are believed to result in a homogeneous dispersion of 20 wt% alumina in Y-TZP. One of the preparation methods (the aalumina method) involves the use of a-alumina instead of transition alumina in order to investigate whether the use of transition aluminas provides any advantage over the use of a-alumina. A fine-grained composite material has to be obtained by pressureless sintering, which offers the possibility of postsintering heat treatments in order to obtain a tetragonal zirconia matrix with optimum mechanical properties. Furthermore, the ceramic grain size

The preparation and characterization of Y- TZP/20 wt% alumina

has to be small in order to obtain a material suitable for hot-forging experiments.

2 Experimental Procedure 2.1 Powder preparation Zirconia ( + 2-2"5 mol % yttria)/12-20 wt % alumina ceramic powders were prepared by four wetchemical techniques: (1) the coprecipitation method, {2) the acetyl acetonate method, (3) the spraying method and (4) the 2-alumina method. The :talumina method also required the use of commercially available fine-grained 2-alumina. The coprecipitation method was used to prepare a composite powder in which the Y-TZP contained 2mo1% yttria. The other preparation methods were used to !~repare composite powders in which the Y-TZP contained 2.5 mol% yttria. In the coprecipitation method the hydroxides were precipitated from one precursor solution in order to obtain an intimate mixture of both phases. Yhis method involved the hydrolysis of a dilute solution (0'4M) of aluminium chloride (Merck, extra pure), yttrium chloride (Cerac, 99'9%t and zirconium chloride (Merck, p.a.) in HCI (0"2M) by adding it slowly to a large excess of NH~OH (Merck, 25%). The hydrolysis was performed in a dispersion turbine reactor. ~ The hydrous gel was washed nine times in water/ammonia mixtures with decreasing amounts of ammonia, using a high-energy disc turbine, in order to remove all chloride. Between each washing step the gel was allowed to settle, after which the clear supernatant liquid was removed. During hydrolysis and subsequent washing the pH remained 11 or more in order to prevent recrystallization of the aluminium hydroxide, whose stability depends on pH. The gel was filtered and wet-milled with ethanol in polyethylene bottles using teflon balls. Subsequently the gel was washed three times with ethanol in the same reactor vessel as described for the water ammonia washing in order to remove free water prior to drying. The gel was dried in air at 12OC, dry-milled in a polyethylene bottle using teflon balls and calcined in air at temperatures ranging from 550C to 1000' C. Calcination involved a heating and cooling rate of 2'5~C/min whereas the ultimate calcination temperature was maintained for 2 hours. Both the acetyl acetonate m e t h o d and the spraying method involved the preparation of yttriastabilized zirconia prior to the addition of alumina. The preparation of the yttria-stabilized zirconia involved the hydrolysis of a dilute solution (0.4M) of zirconium chloride (Merck, p.a.) and yttrium chloride (Cerac, 99.9%) in HC1 (0"2M) in a large excess of ammonia, corresponding to the coprecipit-

499

ation method as already described. The dried gel was used for the acetyl acetonate method, whereas the spraying method required a calcination procedure (550 C: 2 h) prior to the addition of the alumina. The starting point of the acetyl acetonate method was the dispersion of alumina by mixing a hydrous zirconia/yttria gel with a solution of aluminium acetyl acetonate (Merck, >98%) in ethanol. The aluminium acetyl acetonate was dispersed by chemical adsorption of the organometallic compound on the zirconia surface. ~5 In this case the aluminium acetyl acetonate reacts with the hydroxide groups of the hydrous zirconia/yttria gel, forming acetyl acetone as a by-product. ~5 In the acetyl acetonate method 70 g of AI(AcAc)3 was completely dissolved in absolute ethanol (dried on molecular sieves) at a temi~erature of + 80;C. The dried zirconium (yttrium) hydroxide gel was added slowly to this hot solution and mixed thoroughly for several hours. The mixing was performed in a baffled beaker using a dispersion turbine. In this way optimum chemisorption of aluminium acetyl acetohate groups on the hydrated zirconia surface was obtained. The a m o u n t of ethanol was slowly reduced by evaporation. The remaining suspension was wet-milled for two weeks in a polyethylene bottle using teflon balls. The gel was dried for 15 h in air at 8 0 C followed by drying at 120':C for a few hours. The dried gel was milled in a polyethylene bottle using teflon balls and calcined in air at 500cC or 8 5 0 C for 2h in order to decompose the precipitated aluminium acetyl acetonate and to transform the hydroxide into oxide. In one case calcination was performed under a continuous supply of oxygen. In order to establish the importance of dissolving completely the aluminium acetyl acetonate prior to the addition of the hydroxide gel, in one case an ethanol suspension was prepared of aluminium acetyl acetonate and a hydrous zirconia/yttria gel (dried on molecular sieves). In this way relatively large aluminium acetyl acetonate particles may be present from the beginning. The suspension was wetmilled for ten days in a polyethylene bottle using teflon balls and further treated under the same procedure as mentioned previously. The idea behind the spraying method was to increase the degree of dispersion of both alumina and zirconia and to prevent a large local supersaturation of the alumina precursor. This was done by spraying small droplets into dilute ammonia of a suspension containing zirconia and a dilute solution of aluminium chloride. The addition of the precursor to dilute ammonia was performed by spraying instead of adding it dropwise because of the smaller droplet size during spraying. The spraying method involved the preparation of

500

P. den Exter, A. J. A. Winnubst, A. J. Burggraaf

a suspension of yttria-stabilized zirconia (25 g/litre) in a dilute solution (___0.1M) of aluminium chloride (pH 2-3). The zirconia particles were prepared separately as described in this paper. After stirring for 1 h the suspension was subjected to an ultrasonic treatment in order to break down the zirconia agglomerates. In one case the pH of the suspension was adjusted to 1 h by addition of some hydrochloric acid prior to ultrasonification. Directly after ultrasonification the suspension was sprayed through a spraying nozzle into a beaker with dilute ammonia (1 r~; pH > 11) under continuously stirring. The gel was washed twice with dilute ammonia (1 M) in order to remove all chloride. The gel was filtered and wetmilled in ethanol in a polyethylene bottle using teflon balls. Subsequently the gel was washed and dried as previously described. The dried gel was calcined at 500°C for 2 h. The idea behind the a-alumina method was to enclose a fine-grained commercial a-alumina powder (AKP 50, Sumitomo Chemical Co., Ltd) in a hydrous zirconia/yttria gel structure. According to the manufacturer this a-alumina powder has a particle size of 0.1-0-3/~m. The a-alumina method involved the preparation of a suspension of a-Al20 3 (3.83g/litre) in a dilute solution (0-2M) of hydrochloric acid containing zirconium chloride (27.6g/ litre) and yttrium chloride (1.28 g/litre). After stirring the suspension was subjected to an ultrasonic treatment in order to break down all alumina agglomerates. Directly after ultrasonification the suspension was added slowly to a large excess of ammonia, corresponding to the coprecipitation method as already described. The washing procedure was the same as that of the coprecipitation method. The gel was dried in air at 120°C and calcined in air at 550°C for 2h. 2.2 Characterization Oxide powder suspensions used for the preparation of composite powders were subjected for 10 min to ultrasonification (65W) using a Branson Sonifier 450. The particle size distribution of the suspensions was measured prior to hydrolysis using a Horiba LA-500 laser diffraction particle size distribution analyser (particle size range: 0.5-80#m) or a Malvern autosizer 2c (particle size range: 0.2-0.8/~m). Several techniques were used to characterize the chemical and phase compositions of the powders. XRay fluorescence spectrometry using a Philips PW 1410 spectrometer was used for the analysis of the chemical composition of the powders. A Philips PW 1710 X-ray diffract0meter using CuK~ radiation was used to analyse the phase composition of the composite powders by continuous scanning from 20 values of 14 ° to 80 ° with a scan speed of 0.05°/s.

The crystallization behaviour of zirconia during calcination was investigated by differential scanning calorimetry using a Stanton Redcroft DSC 1500. Heating was performed with a heating rate of 10°C/min under a continuous supply of nitrogen using a-alumina as a reference. Powder compacts were obtained by isostatic compaction at 400MPa. Powder compaction behaviour was investigated by measuring the density as a function of the isostatic pressure. Nonisothermal sintering behaviour was investigated using a Netzsch 402E dilatometer at a heating rate of 2.5°C/min. Densification behaviour of the prepared compacts was compared with commercially available Tosoh TZ-3Y20A (Toyo Soda Manufacturing Co., Ltd, Japan). Nitrogen adsorption/desorption isotherms at - 196°C were measured on green compacts using a Micromeritics ASAP 2400 after degassing the compacts at 300°C. The resulting pore size distributions were calculated from the desorption branch of the hysteresis loops. Pores with radii larger than approximately 15 nm were determined by mercuryintrusion porosimetry using a Carlo-Erba Porosimeter (Series 200). Specific surface areas of the powders were measured by means of nitrogen adsorption using the same Micromeritics ASAP 2400. Bulk densities of the compacts were measured by the Archimedes technique (in mercury). Because of the difficulty in determining cell parameters of the transition aluminas all theoretical densities were based on a-alumina and tetragonal zirconia. The ceramic microstructure of polished and thermally etched compacts was examined using a Jeol JSM35CF scanning electron microscope. Ceramic grain sizes were measured using the line intercept technique for measuring grain sizes in two-phase polycrystalline ceramics as described by Wurst & Nelson. 16

3 Results and Discussion 3.1 Powder characteristics It was found that the final chemical composition in almost all preparation methods deviated from 20 wt% alumina in Y-TZP (2-2.5 mol% Y203). The methods in which a suspension of an oxide powder (a-alumina in the a-alumina method and zirconia in the spraying method) was prepared resulted in a slightly lower content of the aluminium oxide which was in suspension (1-3 wt%). This was mainly due to sedimentation of large oxide particles in the suspension which were not used for the hydrolysis of the metal chlorides. Furthermore the acetyl acetonate method resulted in a considerably lower

The preparation and characterization of Y-TZP/20 wt% alumina

501

Table I. Powder characteristics and sintered density of AI20 3 dispersed in zirconia ( + 2-2'5 tool% yttria)

T,.,,t,.

Synthesis method

(' C)

Zirconia phase

Alumina phase

(m2/g)

Coprecipitation

550 800 1000

a t t

u u u

164 90 18

Acetyl acetonate

500 850

a t/(m)

u u

165 60

_+60' 98'

Spraying

500

t

u

135

95

2-Alumina

550

t

z

60-80

98'

t/(m)

~

Tosoh a

SBE.r

16

Sintered density" 96 h 94 h 95 h

98'

a = a m o r p h o u s , m = monoclinic, t : tetragonal, u = unidentified. Phase in parentheses is m i n o r phasc. "Percentage of theoretical density based on ~t-alumina and tetragonal zirconia. bSintered for 2 h at 1600°C. Sintered for 2 h at 1550:C. d Data determined by authors.

alumina content (12-14 wt% A1203). In this case the alumina content proved to depend on the amount of powder (30-50 g) subjected to the heat treatment in order to decompose the aluminium acetyl acetonate. Small powder batches (4-15 g) resulted in the proper alumina content. The adsorption reaction of the aluminium acetyl acetonate might have been insufficient to adsorb all organometallic molecules on the support surface. The remaining part of the dispersed aluminium acetyl acetonate was probably partly evaporated because of insufficiently available oxygen present in the larger batches. Table 1 summarizes some of the powder properties of the composite powders obtained. In all cases the alumina phase is unidentified, except where commercial ~-alumina is used. In the case of low calcination temperatures (500-550°C) either amorphous zirconia or tetragonal zirconia is present. The powders containing amorphous zirconia were

\'x

\ \: "\

600

I

I

I

I

650

700

750

800

TEMPERATURE

850

(~C)

Fig. 1. Typical DSC curves of the crystallization of tetragonal zirconia in powders o b t a i n e d by the acetyl acetonate m e t h o d (--) a n d the coprecipitation m e t h o d ( . . . . . . ). Heating rate: lO°C/min.

obtained by the preparation methods in which both zirconium/yttrium oxide and aluminium oxide were obtained in one calcination step (coprecipitation method; acetyl acetonate method). During calcination at high temperatures (800-1000°C) crystallization of the zirconia phase occurred. In this case the crystallization of amorphous zirconia to the tetragonal structure is clearly visible using differential scanning calorimetry (Fig. 1). The crystallization of zirconia in pure Y-TZP normally takes place at approximately 450°C. ~°'t7 In the presence of aluminium acetyl acetonate the crystallization shifts to 690~C. In the presence of aluminium hydroxide the crystallization temperature shifts to 790°C. Significant variations are obtained in the density after sintering (Table 1). The coprecipitation method results in a density of only 95% after sintering at a rather high temperature (1600°C). The same holds for the spraying method, although the sintering temperature is slightly lower (1550~C). The other preparation methods give a density of 98% after sintering at 1550°C with the exception of the powder prepared by the acetyl acetonate method and calcined at 500°C, which hardly densities. 3.2 Green microstructure Green compacts, after isostatic compaction at 400MPa, obtained by the acetyl acetonate (Tc,lc.: 500C) and spraying method (Tc,~c.: 550°C) showed a narrow pore size distribution with a mean pore radius of 3-4 nm. Only a tail of larger pores was observed in the green compacts with a pore radius in the range of 5-10nm. The pore size distributions resemble those obtained for pure Y-TZP. t° The pore size distributions of the green compacts derived by the coprecipitation method are given in Fig. 2. The compacts calcined at 550°C and 800°C have a bimodal pore size distribution. The compact calcined at 1000°C, not shown in Fig. 2, on the other hand, yielded a very broad pore size distribution

502

P. den Exter, A. J. A. Winnubst, A. J. Burggraaf 02.5

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